1. Field of the Invention
The present invention relates to a diffractive collector for a bar code scanner. In particular, the present invention relates to a wedged-shaped collector which includes a planar front surface and a holographic grating provided on a conical bottom surface.
2. Description of the Related Art
Bar codes are used in a wide variety of applications for retrieving information, such as price, from objects. In this respect, bar code scanners are of widespread use in grocery stores and department stores, for both inventory control and for point-of-sale (POS) transactions.
A bar code normally includes several bar code characters. A bar code character is a group of lines (bars) and spacings that represent a single number or letter. A bar code symbol is a collection of several bar code characters which represent an identification of a particular object. The lines of the bar code can vary, for example, in a range from about 1/18" to 1" in height, and from about 1 to 50 mils in thickness. The spacings between the lines making up the bar code may be of various widths, with the variations in the spacing being one determination of the type of bar code characters making up the bar code.
Typical bar codes are read by illuminating the bars and spacings in a sequential manner, with the bars absorbing light and the background spacings reflecting light. This results in a pattern of reflections and nonreflections that is sensed by a light detecting circuit resident in the bar code scanner. The light detecting circuit provides an input to a digital processor, which interprets the bar code into a digital word.
Various types of apparatuses are used for the light detecting circuits in bar code scanners. For example, a photodiode detector may be used to detect reflected light off of a bar code symbol. The photodiode detector generates an electrical signal indicative of the detected light intensity. The electrical signal is then output to signal processing circuitry, which processes the electrical signal and generates data descriptive of the symbol.
In general, light output by a light sensor within the bar code scanner impinges on a bar code symbol, causing some light to reflect off of the bar code symbol. Some of the reflected light is received by a mirror (typically a curved mirror) within the housing of the bar code scanner (but positioned behind a window that allows for light to pass therethrough). The mirror focuses the received light onto the photodiode detector. Such a system is described in U.S. Pat. No. 5,410,139, entitled "Peak Detecting Bar Code Reader", issued to W. Barkan, which is incorporated herein by reference. In essence, the mirror acts as a "collector" for the reflected light, and outputs the collected light to the photodiode detector. A signal indicative of the amount of the received reflected light is thereby obtained.
Other detailed designs of bar code reading instruments are described in U.S. Pat. No. 5,258,604, which was issued to James Behrens et al. on Nov. 2, 1993, and is entitled "Bar Code Scanner"; U.S. Pat. No. 5,237,161, which was issued to Scott R. Grodevant on Aug. 17, 1993 and is entitled "System for Automatically Reading Symbols, Such as Bar Codes, on Objects Which are Placed in the Detection Zone of a Symbol Reading Unit, Such as a Bar Code Scanner"; U.S. Pat. No. 5,212,371, which was issued to John A. Boles et al. on May 18, 1993 and is entitled "Hand Held Bar Code Scanner with Improved Aiming Means"; U.S. Pat. No. 5,200,597, which was issued to Jay M. Eastman et al. on Apr. 6, 1993 and is entitled "Digitally Controlled System for Scanning and Reading Bar Codes"; and U.S. Pat. No. 5,019,698, which was issued to Jay M. Eastman et al. on May 28, 1991 and is entitled "Bar Code Reading System Having Electrical Power Conservation and Laser Radiation Power Limiting Means." The entire contents of all of the patents and patent applications listed above are incorporated herein by reference. This application and the all of the above-identified patent applications and patents, except for the Barkan patent, are assigned to PSC Inc. (Webster, N.Y.).
Holograms are currently being used for a variety of security-related purposes for smart cards. U.S. Pat. No. 5,101,184, entitled "Diffraction Element And Optical Machine-Reading Device", issued to G. Antes, relates to the use of a hologram on a card, such as a credit card or bank note, so as to provide a measure of security for the card. The invention also relates to a device for reading the information on the card.
U.S. Pat. No. 4,400,616, entitled "Document Card Containing Information In Holographic Form", issued to P. Chevillat et al., relates to a document card provided with secret data contained in the form of a waveguide hologram, and a device for reading the secret data.
One conventional bar code scanner utilizes holograms for outputting light to a bar code symbol. U.S. Pat. No. 5,266,788, entitled "Laser Scanner For Bar Code Reader Having a Transparent Light Guide Plate", issued to K. Yamazaki et al., which is incorporated herein by reference, describes such a conventional scanner.
FIG. 1 shows the use of a hologram for providing a output scan signal according to the Yamazaki invention. In FIG. 1, a transparent light guide plate 23 is integrally formed on a substrate 20, and a scanning beam emission window 25 is formed as a hologram at an upper part of the light guide plate 23. The substrate 20 incorporates a scanning optical system for the scanning beam P. Yamazaki's scanning optical system has a polygonal mirror 31, which is rotated around a rotary shaft 33 by a motor 35. In response to the rotation, the polygonal mirror 31 guides the beam P in a plane within the substrate 20, so that the beam is reflected upward from a lower mirror 37a into the transparent light guide plate 23. The scanning beam P undergoes repeated total reflections inside the transparent light guide plate 23, and is diffracted by the hologram 27 in a predetermined direction and emitted from the emission window 25. The emitting direction is optionally set according to the design of a desired pitch of interference fringes, etc., of the hologram 27. In Yamazaki's invention, the light source of the scanning beam P is a semiconductor laser 41, and the beam P is the incident on the polygonal mirror 31 from the inside of the substrate 20.
FIG. 2 shows another embodiment of Yamazaki's invention, in which a hologram 38 is arranged at a lower end portion of the transparent light guide 23. The hologram 38 diffracts the scanning beam P from the polygonal mirror 31 in a predetermined direction to guide the beam upwardly into the transparent light guide plate 23.
Another conventional bar code scanner utilizes holograms for both outputting light to a bar code symbol, and for receiving light that is reflected off of the bar code symbol. U.S. Pat. No. 5,231,277, entitled "Optical Scanner Using Plannar Reflecting Holograms", issued to H. Aritake et al., which is incorporated herein by reference, describes such a conventional scanner.
FIG. 3 shows a conventional bar code scanner according to Aritake, in which a rotary polygon mirror is employed as a laser beam scanning means and strip holograms of the transmission type are employed for a reading window. The reading window 10 is composed of three transparent substrates 11, 12 and 13 having strip holograms 1la, 12a and 13a, respectively, of the transmission type formed in different directions from each other and adhered in layers to each other such that the strip holograms 11a, 12a and 13a extend in an intersecting relationship to each other. Located below the reading window 10 are a scanning pattern generating mirror means 14 which consists of three side mirrors 15, 16 and 17, a concave mirror 18 having a through-hole 18a formed therein, a bottom mirror 19 disposed in parallel to the reading window 10, a photo-detector 20, a mirror 21, and a polygon mirror 23 having five reflecting faces and connected to be driven to rotate by a motor 22. A laser beam output a light beam that is projected onto the reading window 10 by way of the mirrors 15, 16 and 17 and the bottom mirror 19 to successively scan the three strip holograms 1la, 12a and 13a. The laser beams diffracted by any of the strip holograms 11a, 12a and 13a are projected as scanning lines of predetermined spacings, and a desired scanning pattern is formed as a result.
Also, as shown in FIG. 4, signal light from a bar code affixed to a commodity is diffracted by the reading window 10 and introduced to the bottom mirror 19, and then reflected successively by the bottom mirror 19, side mirror 16, and polygon mirror 23 so that it is introduced to the concave mirror 18. The scattered light is condensed and reflected by the concave mirror 18 and then introduced by way of the mirror 21 into and detected by the photo-detector 20.
The above-mentioned device, however, requires three horizontally divided side mirrors incorporated below the reading window, in which the reading window itself has three transparent substrates that form respective strip holograms. In view of this, Aritake discloses a system as shown in FIG. 5, in which a hologram 34 of the reflection type is used for generation of a scanning pattern. The hologram 34 is used with a concave mirror 18 having a through-hole 18a formed therein and having a curved reflecting face, a photo-detector 20, a reflecting mirror 21, a polygon mirror 23' having six reflecting faces, and connected to be driven by a motor 22, a laser tube 24, a beam shaper 25 and another reflecting mirror 26, that are disposed below a reading window in a substantially similar fashion to the device shown in FIG. 3.
The reaction type hologram 34 comprises three strip holograms 35, 36 and 37 of the reflection type having different diffraction directions from each other, and which are disposed in a vertically overlapping relationship in the same vertical plane. Each of the strip holograms 35, 36 and 37 has interference fringes formed therein such that they may diffract a scanning beam introduced thereto from the polygon mirror 23' so as to irradiate the scanning beam upon strip holograms (not shown) of the transmission type of the reading window 30 from below. In producing a hologram of the reflection type, a reference beam is introduced to one face of a photographic plate in the same direction with a laser beam which is to be used upon reproduction of a hologram while an object beam is introduced to the other face of the photographic plate in such a direction that a desired scanning line may be formed on the reading window upon reproduction of the hologram. With this method, a hologram having an arbitrary diffraction angle can be produced.
FIG. 6 shows another embodiment of the use of a hologram in the Aritake patent. Scattered signal light S1 from the bar code 50a advances reversely along the substantially same light path with the incident beam and is introduced to the hologram 55 of the reflection type having a concave mirror. Diffracted light S2 from the hologram 55 is condensed by way of a mirror 21 to a photo-detector 20 so that the bar code information is read by the photo-detector.
U.S. Pat. No. 5,714,750, issued to Jay Eastman et al., and assigned to PSC Inc., which is incorporated herein by reference, describes the use of concentric, circular-shaped holographic gratings which provide the means for guiding return light to a photodetector positioned to a central point with respect to the concentric circular gratings. As a result, a miniaturized collector for a bar code scanner can be constructed.
U.S. Pat. No. 5,786,585, issued to Jay Eastman et al., and assigned to PSC Inc., which is incorporated herein by reference, describes the use of diffraction gratings on a collector of a bar code scanner. The diffraction gratings vary in width depending upon where their location on a front surface of a substrate. Due to the diffraction gratings, the return light is focused onto a particular area, where a detector is situated.
It is desirable to utilize a hologram in a collector for receiving light reflected off of a scanned bar code symbol, such that the collector is compact in size and not as complex as the conventional devices described above.